1. Field of the Invention
The present invention relates to a multiple rotating absolute encoder.
2. Related Background Art
FIG. 4 is a block configuration diagram of a conventional multiple rotating absolute encoder. The illustrated multiple rotating absolute encoder is equipped with an encoding plate provided on the rotation axis, a detection unit 10 for detecting the number of rotations, and a detection unit 13 for detecting an absolute address in one rotation.
On the encoding plate, there are formed a track for detecting the number of rotations of the encoding plate, and a track for detecting the absolute address in one rotation of the encoding plate.
The detection unit 10 for detecting the number of rotations issues an output in order to obtain data Q on the number of rotations from a detection signal which reads the track for detecting the number of rotations.
The detection unit 13 for detecting the absolute address in one rotation issues an output in order to obtain data R on the absolute address in one rotation from a detection signal which reads the track for detecting the absolute address in one rotation.
Further, the multiple rotating absolute encoder is equipped with a data output circuit 16 for receiving the data Q on the number of rotations from the detection unit 10 and the data R on the absolute address in one rotation from the detection unit 13. The data output circuit 16 internally latches the data Q with the data R so as to output an external output signal s corresponding to the absolute position of encoding plate.
This external output signal s contains both data Q and data R.
The detection unit 10 contains a magnetic encoder unit 11 and a counter circuit 12 for counting the number of rotations.
The magnetic encoder unit 11 is constituted by a magnetic pattern which is formed on the encoding plate and serves as the track for detecting the number of rotations mentioned above, and an MR sensor (magnetoresistance sensor) which reads the magnetic pattern and outputs incremental two-phase (the phase A and the phase B) signals l and m.
The counter circuit 12 calculates the data Q from the incremental two-phase signals l and m output from the magnetic encoder unit 11, and outputs said data Q to the data output circuit 16 via a data bus 17.
Note that the detection unit 10 for detecting the number of rotations is arranged to be driven by an external battery or a bulk capacitor inside the encoder (hereinafter simply called the "external battery") even when the main power source is turned off so that it can be operated even when a supply of electric power is stopped.
In order to increase an operation time of the external battery, i.e., a backup time, it is required to decrease the electric power consumed by the detection unit 10. Therefore, a magnetic encoder which consumes a smaller amount of electric power is generally used as a detection unit for detecting the number of rotations for the multiple rotating absolute encoder.
The detection unit 13 for detecting the absolute address in one rotation contains an optical encoder unit 14 and a data conversion circuit 15.
The optical encoder unit 14 reads the track for detecting the absolute address in one rotation of the encoding plate and outputs a data signal (absolute signal) n on the absolute value in one rotation and incremental two-phase (the phase A and the phase B) signals o and p in one rotation.
The data conversion circuit 15 calculates the data R on the absolute address in one rotation based on the data signal n and the incremental two-phase signals o and p in one rotation, output optical encoder unit 14 and outputs said data R to the data output circuit 16 via a data bus 18.
Note that the detection unit 13 for detecting the absolute address in one rotation is arranged to be driven when the main power source is turned on, and to stop its operation when the supply of the electric power is stopped.
In the conventional multiple rotating absolute encoder as mentioned above, it is required to internally latch the data Q with the data R without a time lag in the data output circuit 16. If any time lag is generated, a temporal mismatch arises between the data Q and the data R so that the absolute position of the encoding plate may be detected on the basis of an erroneous number of rotations by plus or minus one rotation, as the case may be.
Such an error in the number of rotations spoils the detection accuracy of the absolute position of the encoding plate and becomes a fatal defect for the multiple rotating absolute encoder.
FIGS. 5 and 6 are views for explaining a time lag between the data Q and the data R.
In FIGS. 5 and 6, the data R obtained in the optical encoder unit is compared with the data Q obtained in the magnetic encoder unit temporally at the time of low-speed rotation and at the time of high-speed rotation, respectively. Note that the data R obtained in the optical encoder unit relates to the number of rotations obtained by using the data on the number of rotations which is detected in the magnetic encoder unit when the supply of the electric power is resumed as an initial value and synthesizing this initial value and the data on the absolute address in one rotation of the optical encoder unit.
As shown in FIG. 5, since a response time of the magnetic encoder unit has no influence at the time of low-speed rotation (this response time indicates a time from a detection of a change in magnetism by the MR sensor to an output of the data on the number of rotations), there arises no time lag between the data on the number of rotations of the magnetic encoder unit and that of the optical encoder unit.
However, as shown in FIG. 6, owing to a delay in the response time of the magnetic encoder unit, areas 50 and 51, which are not temporally matching with the two pieces of data on the number of rotations, are revealed at the time of high-speed rotation. For example, in the area 50, though the data on the number of rotations of the optical encoder unit is n, that of the magnetic encoder unit is n-1. Such time lag of two pieces of data oh the number of rotations indicates that the data on the number of rotations temporally deviates from the data on the absolute address in one rotation in the magnetic encoder unit.
More specifically, since the response time of the optical encoder unit is fast, as compared with that of the magnetic encoder unit, when a rotation of the rotation axis of the absolute encoder, that is, a rotation of the encoding plate, becomes of high speed, a time lag is revealed between the data on the number of rotations and the data on the absolute address in one rotation. In other words, the above-mentioned time lag can not be actually avoided unless the rotation speed of the rotation axis of the absolute encoder is made smaller. As a result, there arises an inconvenience that the rated rotation speed of the absolute encoder is limited by a delay in the response time of the magnetic encoder unit (that is, the phase delay). Reducing a delay in the response time of the magnetic encoder unit in order to avoid the time lag could also be considered. However, in this case, it is required to increase an amount of power to be consumed by the magnetic encoder. As a result, it becomes difficult to ensure a sufficient backup time of the magnetic encoder, and a down-sizing of specifications such as a reduction of the time for securing the data on the number of multiple rotations becomes necessary.